6O WATER 



Bacteriological Testing of Filter-Beds. 



The testing of the efficient working of sand filter-beds is entirely 

 a bacteriological matter. All filter-beds should be systematically 

 tested to ascertain the percentage of bacteria removed. 



The purification by filtration through sand is only to a small 

 extent mechanical ; it is mainly vital. This vital or biological 

 action is due to the formation, which takes place after a few 

 days of working, of a gelatinous layer on the surface of the 

 sand. This is composed partly of suspended matters and partly 

 of bacteria, algae, and other lowly forms of vegetable life, derived 

 from the water filtered. Such a filter-bed is capable under suit- 

 able conditions of removing the vast majority of bacteria from 

 the water. The percentage removed depends upon a number 

 of factors, of which the following are the most important : The 

 rate of filtration, the age of the filter, the depth and size of the 

 sand particles, the kind of filtration i.e., whether intermittent or 

 continuous and the nature of the water filtered. 



Of these, the first two are the most important. If water is 

 passed rapidly through a filter, the percentage of bacilli removed 

 will diminish. Koch recommended that the rate should not 

 exceed 4 inches per hour. The rate of filtration must however 

 be largely governed by the quality of the water being filtered. 

 The age of the filter is in the main a question of the thickness 

 of the gelatinous layer. The percentage of organisms removed 

 should be 98 or more. 



To estimate the bacterial efficiency, or percentage of organisms 

 removed by the filtration, gelatine and agar plates are made 

 from the water before and after filtration, and examined and 

 counted in the ordinary way. Each bed should be separately 

 tested. The rate of filtration and all particulars must be 

 recorded at the same time. The percentage removal of B. coli 

 should also be ascertained, and is on the whole a better test of 

 the efficiency of filtration than ordinary bacterial enumerations. 



